We propose to synthesize potent and stable opioid peptide analogs, designed to be i) highly selective delta antagonists without mu antagonist properties, ii) mixed mu agonists/delta antagonists and iii) structurally rigid mu-selective agonists. The compounds to be developed might also be useful for the further demonstration of putative mu-, delta- and kappa-opioid receptor subtypes and as selective ligands for such subtypes. Highly receptor-selective opioid peptide agonists, antagonists or mixed agonists/antagonists are needed in studies aimed at correlating specific opioid effects with a distinct receptor class or subclass as well as for the development of potential therapeutic agents showing minimal side effects. to reach these goals, we use an interdisciplinary approach incorporating peptide synthesis, conformational investigations and extensive pharmacological characterization. The peptide analog design will be based on various principles, including substitution of natural and artificial amino acids, peptide bond replacements and introduction of conformational constraints. Specific aims include: a) synthesis of highly potent and stable analogs of the delta-selective prototype antagonists H-Tyr-Tic-Phe-Phe-OH (TIPP) and H-Tyr-Tic-Phe-OH (TIP) recently discovered in our laboratory; b) development of mixed mu agonists/delta antagonists based on structural modification of H-Tyr-Tic-Phe-Phe-NH2 (TIPP-NH2) or H-Tyr-cyclo[-D-Orn-1- Nal-D-Pro-Gly-] and on the preparation of peptide dimers; and c) synthesis of analogs of the mu-selective cyclic dermorphin peptide H-Tyr- D-Orn-Phe-Asp(or Glu)-NH2 containing additional conformational constraints at the Tyr1- and/or Phe3- residue or in the peptide ring structure; d) conformational studies by molecular mechanics and molecular dynamics techniques, and by 1- and 2-dimensional NMR spectroscopic methods; e) determination of receptor affinities and selectivities of the new compounds in binding assays based on displacement of mu-, delta- and kappa-selective radioligands from rat or guinea pig brain membrane binding sites; f) evaluation of agonist and antagonist properties of the analogs in bioassays based on inhibition of electrically evoked contractions of the guinea pig ileum and of the vasa deferentia of the mouse, rat and rabbit; g) examination of the stability of the compounds against enzymatic degradation; and h) tests of the analgesic properties of the analogs using the mouse writhing and hot plate assays.